Mosaic electrode structure



Nov. 26, 1940.

MOSAIC ELECTRODE STRUCTURE Filed May 19, 1938 wvnv r02 650.465 874MB PRC/4i FREEMAN ATTORNEY G. s. P. FREEMAN I, 2, 22,940

Patented Nov. 26, 1940 rarest ore-ice MGSAIU ELECTRODE STRUCTURE .George Stanley Percival Freeman, Hammersmith, 7 London, England, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, Eng land, a company of Great Britain Application May 19, 1938, Serial No. 203,744

lnGreatBritainMay 28, 1937 3 Claims.

This inventionrelates to mosaic electrode structures 'for use in cathode ray television and similar apparatus. The invention is concerned with the kind of mosaic structure in which a 5 multiplicity of mutually insulated metallic elements are formed on the surface of an insulating carrier or base plate. I i In television transmitting systems use is sometimes made of a cathoderay tube transmitting apparatus comprising a mosaic electrodestructure consisting of a multiplicity of mutually insulated photo-sensitive elements upon which an image of an object for transmission is projected the individual elements acquiring electrostatic charges according to the intensity of the incident light image and the elements are periodically restored toa datum value on being scanned by a cathode ray beam, or in some cases by a light beam, restoration of the elements to the datum potential generating in a conductive signal plate associated with the mosaic electrode picture signals suitable for transmission.

In other forms ofcathode ray television transmitting tubes an electron image of an object for transmission is projected onto a mosaic structure, which in this case is not rendered photo-sensitive, the electron image releasing secondary electrons on impact with themosaic screen, the release of secondary-electrons causing the elements of the screen to acquire electrostatic chargeswhich-are periodically restored to-a datum value by scanning with a suitable scanning beam.

Heretofore in the manufacture of mosaic elec-' trode structures it has usually been the practice to coat a supporting base plate, usually of mica, with a'layer of silver, and to heat the base plate with the silver layer to a sufiicient degree to cause the silver to aggregate and assume a globular formation, the globules so formed aiiording a multiplicity of minute insulated elements which, when used in thefirst-mentioned type of cathode ray tube, are rendered photo-sensitive.

The manufacture of mosaic electrode structures,

in'this manner is fully referred to in'the specification of British Patent No. 407,521.

The process of making mosaic electrode structures as heretofore adopted has necessitated heat treatment of the silver-layer in order to cause aggregation into minute globules.

his the object of the present invention to provide an improved method of making mosaic electrode structures in which the necessity for heating the electrode for the production of the mosaic I formation is avoided.

According to thepresent invention a method of making a mosaic electrode structure is provided in which a suitable metal is applied to an insulating base-plate or support in a sufiiciently thin layer that the particles of metal deposited on the .base-plateare adequately insulated from one another whilst a sufiicient number of particles is applied to the surface as eiiectively to constitute amosaic electrode without the necessity of heating the metalliclayerto cause aggregation into minute globules. The metal employed for coating the base-plate, which is preferably a sheet of mica, will in most cases .be silver since this metal is found to be particularly suitable for use in the invention and can bereadily oxidised when it is desired to prepare a photo-sensitive mosaic electrode since the silver-oxide has great affinity for the usual-photo-sensitive material employed, namely, caesium.

It is practically impossible to measure in linear dimensions the thickness of the layer which is sufficient to afiord the elements Without the insulation between the elements being'reduced to a degree'lower than that considered necessary in practice. However, in the following specific description of the invention an indication will be given of the manner of forming a surficiently thin layer'for use in accordance with theinvention.

' In order that the saidinvention may be clearly understood and readily carried into effect, the same will now-be more fully described with ref erence to the accompanying diagrammaticdrawing which illustrates one form of cathode ray television transmitting tube in which a photosensitive mosaic electrode in accordance with the invention can be employed. a p

The television transmitting tube shown in the drawing "comprises an evacuated envelope l in which there is suspended in known manner a mica support or base-plate 2 having on one side a conductive signal'plate 3 and on'the other side a mosaic structure indicatedby the dotted line 4.

.In a-tubular portion 5 of the envelope there is provided a cathode 6 associated with other electrodes in known manner'which serves to focus a beam of electrons emanating from the cathode 6 for the purpose of scanning the mosaic screen 4, the beam being deflected in known manner by. suitable coils or electrostatic deflection plates. In the form of apparatus shown in the drawing the'mosaic structure is rendered photo-sensitive and on projecting an optical image onto the mosaic structure througha "suitable optical systemindicated at 1, the elements of the mosaic structure emit photo-electrons causing theelements to become electrostatically charged and these charges are periodically restored to a datum potential by scanning with the electron beam,

restoration of the elements to the datum potential generating in the signal plate 3 picture signals which are developed across a resistance 8 and are fed to an amplifier for amplification prior to transmission, the first valve of the amplifier being indicated at 9. In the manufacture of the electrode structure 4, a layer of silver is usually applied to the support 2 and the support is subsequently heated to cause the silver to assume aglobular formation, as stated above. In accordance with the present invention a sufliciently thin layer of a suitable metal, preferably silver, is applied to the mica support 2, the thinness of the layer being such that the particles of the metal are adequately insulated from one another and are sufiicient in number so as to function effectively as a mosaic structure.

It has been found, as a result of experiment, that with a certain thickness of coating 21 critical condition is reached. At this thickness the superficial specific resistance of a silver layer jumps suddenly from an unmeasurably high .value to a comparatively low. value (e. -g., 10 to 10- Sl/cmP), the latter value depending greatly on the surface condition of the base-plate. At this critical value the film 'of metal applied to the surface is either substantially continuous or discontinuous, it being appreciated that a discontinuous film is one that is required for the pur-- pose of the present invention. In most cases the insulating support plate will be made of mica and by using this material or some other form of transparent insulating plate, the correct amount of metal to be applied to the base-plate can be. readily ascertained. With such a base-plate it is found that when the transparency of the baseplateis reduced by the metallic coating to approximately '-80% of its original value, a coating of silver sufiiciently thin for the purpose in view will be provided. The silver is preferably applied to the surface by evaporating the silver thereon in vacuo. The transparency of the mica can be readily ascertained by projecting light through the mica onto a photo-cell and connecting the photo-cell to a microammeter. The reading of the microammeter is noted when light is projected through the untreated mica base-plate and when during treatment the reading of the microammeter has fallen to approximately 70-80% of its original value a coating of the required thinness will have been applied to the mica. v

If necessary, the support can be weighed before and after the evaporating process and the amount, by weight of silver, necessary for the layer, can be ascertained, and for the manufacture of subsequent electrodes one or more tungsten or other suitable filaments or coils may be plated with the required'weight of silver, the tungsten coils being then used for the manufacture of furtherelectrodes the silver depositedon the tungsten filaments or coils being evaporated onto the support transparent or otherwise by heating the filaments.

It may bementioned that the critical thick: ness of layer above mentioned, for silver,-occurs when the transparency of themetal-film to trans-. mitted light is approximately 70 80%, and hence, providing this critical value is not reached, mosaic electrodes can be constructed sufilcient for practical purposes Without the necessity of heat: ing the layer to cause aggregation, as has here- 'tofor'e been considered necessary.

For the purpose of obtaining a layer of uniform thickness the apparatus described in the specification of British Patent No. 480,946 may be used.

When the required amount of silver necessary to form the thin layer has been determined in the manner described above, the mosaic electrodes can be manufactured in situ within the evacuated envelope I. For the production of a photo-sensitive mosaic screen the usual steps of oxidising the layer and admitting caesium to sensitise the screen may be adopted. If desired, after sensitising, 'an additional thin layer of silver may be deposited, as referred to in the specification of British Patent No. 480,946.

Where th invention is applied to the type of television transmitting tube in which an electron image is projected onto a mosaic structure, the step of sensitising the mosaic screen will of course .be omitted.

It will be appreciated that the degree of insulation necessary between the elements of the mosaic structure depends upon the conditions under, which the apparatus is operated. The specific description for the production of a mosaic elec, trode in accordance with the invention has been given on the assumption that the structure will be applied to a television transmitting tube in which 25 picture frames per second are scanned. In this case, the insulation between the elements is such that there is substantially no leakage of charge from one element to the other in a frame period, i. e., /2 of a second.

I claim: 7 v

1. A method of fabricating a mosaic electrode structure having an insulating-base member and a signal plate on one side thereof which comprises the steps of ascertaining the light conductivity of the base, applying a suitable substantially non-photoelectric metal to the insulating base plate surface opposite the signalplate as a layer of non-contiguous particles of the metal, and continuing the application of the metal to the insulating base plate until the light conductivityof the base and layer of particles is reduced in the order of 20 to 30 percent of the original light conductivity of the base so that a mosaic surface of substantially non-contiguous and electrically isolated metal particles functioning as a multiplicity of condenser elements in cooperation with the signal plate.

2. A method of fabricating a mosaic electrode structure having an insulating base member anda signal plate on one side thereof which comprises the steps 'of determining the light conductivity of the base member, evaporating metallic, silver onto the insulating base-surface'opposite the signal plate as a layer of discrete non-contiguous particles of the evaporatedsilver, th'epaiticles being electrically insulated one from .the:

other, and continuing the appuca'uss of .the evaporated silver to the insulating jbase'mernber until the light conductivity of the base member with the layer of particles is reduced by 20..to 30.

percent of the original lightlconductivity of the,

base member so that a sufficient numberof'metallic particles are applied to constituteeife'ctive l y a mosaic electrode wherein the receptive particles thereof are substantially; non-contiguous and electrically isolated one from the other. v

73 A method of fabricatinga mosaic electrode structure having a light conductive insulating base member which comprises the steps, of ascertaining the light transparency of the. base mem-: ber, applying a suitable substantially non-jphotoelectric metal to th surface of :the insulating percent of the original light transparency of the' base member, at which tim a suflicient number of metallic particles are applied to constitute effectively a mosaic electrode wherein the receptive particles thereof are substantially non-contiguous 5 and electrically isolated one from the other.

GEORGE STANLEY PERCIVAL FREEMAN. 

